1. Field of the Invention
The present invention relates to a method and apparatus for machining V grooves, which is useful for machining a mold for molding e.g. a lenticular lens sheet or a cross lenticular lens sheet.
2. Background Art
The recent advancement of machine control technology has realized ultraprecision machining with a lathe, and it has become possible to machine even with a lathe a mold for molding an optical lens. For example, the applicant has proposed a vertical lathe capable of machining a mold for molding a Fresnel lens (Japanese Patent Laid-Open Publication No. 2004-358624). The vertical lathe can machine with high precision V-shaped lens grooves of a mold for molding of a Fresnel lens.
Molding a lenticular lens sheet, a cross lenticular lens sheet, a prism sheet, etc., which are for use in a backlight of a liquid crystal panel, by extrusion molding is being studied these days.
The applicant has proposed precision roll turning lathes for machining a roll for use in extrusion molding of such a lens sheet (Japanese Patent Applications Nos. 2006-130066, 2006-135560, 2006-156388, 2006-165144 and 2006-166404).
FIG. 5 shows the surface of a mold for molding a cross lenticular lens sheet. As shown in FIG. 5, a pattern of minute four-sided pyramids densely arranged in a matrix is formed in the surface of the mold. Such a pyramid pattern can be formed by creating longitudinal and lateral V grooves at a predetermined pitch in the surface of a workpiece such that the both grooves intersect.
The conventional V-groove machining for forming four-sided pyramids highly depends on the machine accuracy, involving the problem that the machine accuracy directly reflects the shape accuracy of the resulting four-sided pyramids.
Thus, poor mechanical accuracy will give rise to variation in the depths of V grooves as cut by a cutting tool, resulting in the formation of four-sided pyramids having vertices of non-uniform heights. The machined mold will be inappropriate for use in molding of a high-precision cross lenticular lens sheet.